The present invention relates to blood glucose monitoring devices, and specifically to a miniaturized blood glucose monitoring system which may be inconspicuously worn by a patient and provides a relatively continuous monitoring of the blood glucose level while causing a minimal invasion into the body.
For diabetic patients, the maintenance of blood glucose levels within a very narrow range (about 3.5-6.5mM) is extremely important, since the brain uses glucose as virtually its only source of energy, and low glucose levels (hypoglycemia) cause mental confusion, and if sustained, coma and death. On the other hand, high blood glucose levels (hyperglycemia) cause unpleasant short term symptoms, such as frequent urination and thirst, and in the long term are probably a major factor in the development of tissue damage in the blood vessels, eyes, kidneys, nerves, etc.
For most diabetics, the injection of insulin is required to maintain blood glucose at acceptable levels. Conventional treatment takes the form of injections of insulin into the subcutaneous tissues of the arms, legs, abdomen or buttocks. Normoglycemia is difficult to achieve in diabetics because insulin injections do not mimic nondiabetic insulin secretion patterns sufficiently closely and there is no feedback control of insulin delivery rates according to the prevailing glucose level. In the last few years there has been an intensive effort to improve metabolic control in diabetics by developing a more physiologically related strategy of insulin administration. Research efforts in this area have been focused on the development of an implantable and inconspicuous glucose sensor for the continuous monitoring of glycemic control. Much of the work to date pertains to sensors which monitor the electric current produced by a reaction of blood glucose with an enzyme such as glucose oxidase.
In such conventional amperometric glucose sensors, the generated hydrogen peroxide or consumed oxygen produced by the catalytic oxidation of glucose by immobilized glucose oxidase is measured. An alternative strategy is to incorporate a redox mediator such as ferrocene in the sensor so that electrons are transferred directly from the prosthetic group of reduced glucose oxidase to a base electrode. The advantages of this latter approach include the fact that the reaction does not require the presence of molecular oxygen and is therefor less prone to changes in tissue oxygen levels.
The design objectives of a continuous glucose monitoring system would include miniaturization for ease of patient comfort and mobility, accuracy, ease of use by younger patients, and minimal bodily invasion, such as at or just below the skin surface.
A miniaturized glucose monitoring system employing subcutaneous needle type glucose sensors has been proposed; however, due to the decrease in accuracy of the platinum needle sensors after about 3 days, the long term use of such a system has been impractical.
Thus, there is a need for a blood glucose monitoring system which is miniaturized to the extent that it can be worn comfortably and inconspicuously by a patient, which is accurate and which provides the capability of readily exchanging needle type glucose sensors and accurately connecting them to a monitoring unit by patients of all ages.